1. Technical Field
This document relates to an organic light emitting diode display device compensating the threshold voltage of a driving thin film transistor (TFT).
2. Discussion of the Related Art
With the development of information society, the demand for various types of display devices for displaying an image is increasing. Various flat panel displays such as a liquid crystal display (LCD), a plasma display panel (PDP), and an organic light emitting diode (OLED) have been widely used in recent years. Among the flat panel displays, the organic light emitting diode display device are driven at a low voltage, are thin, have a wide viewing angle and a quick response speed.
A display panel of the OLED display comprises a plurality of pixels arranged in a matrix form. Each of the pixels comprises a scan thin film transistor (TFT) for supplying a data voltage of a data line in response to a scan signal of a scan line and a driving TFT for adjusting the amount of the current supplied to an organic light emitting diode in accordance with a data voltage supplied to a gate electrode. The drain-source current Ids of the driving TFT supplied to the organic light emitting diode can be expressed by following equation:Ids=k′·(Vgs−Vth)2  (1)where k′ represents a proportionality coefficient determined by the structure and physical properties of the driving TFT, Vgs represents the gate-source voltage of the driving TFT, and Vth represents the threshold voltage of the driving TFT.
The drain-source current Ids of the driving TFT depends upon the threshold voltage Vth of the driving TFT. However, the threshold voltage Vth of the driving TFT of each of the pixels may have a different value due to a shift in the threshold voltage Vth caused by degradation of the driving TFT. Hence, the current Ids supplied to the organic light emitting diode differs from pixel to pixel even if the same data voltage is supplied to each of the pixels. Accordingly, the luminance of light emitted from the organic light emitting diode of each of the pixels may differ even if the same data voltage is supplied to each of the pixels. To solve this problem, various types of pixel structures for compensating the threshold voltage Vth of the driving TFT have been proposed.
FIG. 1 is a circuit diagram showing a part of a conventional diode-connected threshold voltage compensation pixel structure. FIG. 1 depicts a driving TFT DT supplying the current to an organic light emitting diode and a sensing TFT ST coupled between a gate node Ng and drain node Nd of the driving TFT DT. The sensing TFT ST allows for a connection between the gate node Ng and drain node Nd of the driving TFT DT during a threshold voltage sensing period of the driving TFT DT so that the driving TFT DT functions as a diode. In FIG. 1, the driving TFT DT and the sensing TFT ST are illustrated as N-type MOSFET (Metal Oxide Semiconductor Field Effect Transistors).
Referring to FIG. 1, the gate node Ng and the drain node Nd are coupled during the threshold voltage sensing period in which the sensing TFT ST is turned on, thereby the gate node Ng and the drain node Nd are in a floating state at substantially the same potential. The floating state refers to a state in which no voltage is supplied to a node, so the node on the floating state affects a voltage change of an adjacent node easily. If a voltage difference Vgs between the gate node Ng and a source node Ns is greater than a threshold voltage, the driving TFT DT forms a current path until the voltage difference Vgs between the gate node Ng and the source node Ns reaches the threshold voltage Vth of the driving TFT DT, and as a result, the voltage of the gate node Ng and the voltage of the drain node Nd are lowered. However, if the threshold voltage Vth of the driving TFT DT is shifted to a negative voltage, the voltage difference Vgs between the gate node Ng and the source node Ns cannot reach the threshold voltage Vth of the driving TFT DT, even if the voltage at the gate node Ng goes down to the voltage at the source node Ns, because the threshold voltage Vth of the driving TFT DT is lower than 0 V. Consequently, if the threshold voltage Vth of the driving TFT DT is shifted to a negative voltage, it is impossible to sense the threshold voltage Vth of the driving TFT DT correctly. A negative shift refers to shifting the threshold voltage Vth of the driving TFT DT to a voltage lower than 0 V when the driving TFT DT is implemented as an N-type MOSFET. The negative shift usually occurs when a semiconductor layer of the driving TFT DT is formed of an oxide.